Smart lunged at Smith after the Cavs player shoved an airborne Al Horford in the back as the Celtics took a 2-0 lead in the best-of-seven series on Tuesday at the TD Garden.

Smith was later issued with a flagrant foul over the push, which sent Horford crashing to the floor. Smart meanwhile needed to be restrained after angrily confronting Smith over the incident.

Smart later told reporters he had felt moved to defend his team-mate Horford.

"You know, and I just looked at it, Al is a defenseless person. He's in the air. He can't control how his body goes, and he's not even looking, and you go and take two hands to the back, that's a dirty shot," Smart said following Boston's 107-94 win.

"You just can't allow that to keep happening. That's not the first time JR has done some dirty stuff, especially playing against us. He's known for it, especially playing against us. We know that.

"So you know, it's like a bully, you keep letting a bully keep picking on you, he's going to pick on you until you finally stand up, and that's what I tried to do."

"One of my guys was down, and I took offence to it," added Smart, who produced a superb defensive display to help contain LeBron James and the Cavaliers on Tuesday.

Smart, 24, said his refusal to back down was the result of his upbringing as the youngest of four brothers.

"That's me. That's how I was raised," Smart said. "I'm the youngest of four boys. My whole life I had to fight. I had to get down and do things in order to secure my spot in the household.